In addition to providing printed telephone directories telephone companies provide telephone directory assistance services. Users of these services call predetermined telephone numbers and are connected to directory assistance operators. The operators access directory databases to locate the directory listings requested by the users, and release the telephone numbers of those listings to the users.
Because telephone companies handle a very large number of directory assistance calls per year, the associated labor costs are very significant. Consequently, telephone companies and telephone equipment manufacturers have devoted considerable effort to the development of systems that reduce the labor costs associated with providing directory assistance services.
In a typical directory assistance system the caller is first prompted to provide locality information, in other words to specify in what area resides the business or individual whose telephone number he seeks. If valid speech is detected, the speech recognition layer is invoked in an attempt to recognize the unknown utterance. On a first pass search, a fast match algorithm is used to select the top N orthography groups from a speech recognition dictionary. In a second pass the individual orthographies from the selected groups are re-scored using a more precise likelihood computation. The top orthography in each of the top two groups is then processed by a rejection algorithm that evaluates if they are sufficiently distinctive from one another so the top choice candidate can be considered to be a valid recognition.
The signal processing operation described above is based solely on an acoustic analysis of the spoken utterance. This sometimes may not enable the system to make a resolution. Indeed, the wide variety of accents that exist in the population and, more particularly, the manner with which individuals formulate requests results in situations in which correct word recognition cannot be made solely on the basis of acoustic match. Thus, there is a need in the industry to provide a speech recognition system that utilizes additional elements of information that when combined with the acoustical analysis of the spoken utterance enables to improve the speech recognition accuracy.